Implement parallel delete

This is first patch in the series to implement parallel delete
and it includes following changes:

1. changing Stack.delete to use stack_task
2. In Resource.delete polling for check_delete_complete
3. Updating unit tests to use TaskRunner for resource.delete and
   resource.destroy

Blueprint parallel-delete

Change-Id: I8a96b1a956388372c29f9f45445f397467667110
parallelize volume deletion

This is second patch in the series to implement parallel delete
In this patch, parallelizing volume deletion and updating relevant
tests

Blueprint parallel-delete

Change-Id: Iaded5951618c2ac52c26d6d6437f152c658ad9cf
Merge "Implement parallel delete"
Merge "parallelize volume deletion"